final db structure

db/migrate/20170403174029_create_sale_orders.rb

@@ -0,0 +1,10 @@
+class CreateSaleOrders < ActiveRecord::Migration[5.0]
+  def change
+    create_table :sale_orders do |t|
+      t.references :sale, foreign_key: true
+      t.references :order, foreign_key: true
+
+      t.timestamps
+    end
+  end
+end